Semiconductor laser device

ABSTRACT

Disclosed is a semiconductor laser device capable of minimizing the spot diameter of a laser light and also capable of improving the transmittance of light passing through a fine aperture. The semiconductor laser device comprises a light absorption film provided with a fine aperture on the outside of the light-emitting surface of the semiconductor laser element. The aperture is formed such that the aperture width W 1  in a direction parallel to the polarizing direction of the laser light is smaller than half the oscillation wavelength of the semiconductor laser element, and the aperture width W 2  in a direction perpendicular to the polarizing direction is larger than the aperture width W 1 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.09/749,801, filed Dec. 28, 2000, now U.S. Pat. No. 6,587,494.

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 11-373067, Dec. 28, 1999, theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor laser device,particularly, to a semiconductor laser device provided with a lightabsorption film having an aperture on the outside of a light-emittingsurface.

In order to improve the recording density of an optical disc, requiredare a light source and an optical system capable of converging the laserlight on a minimal spot. In general, the diffraction-limited spotdiameter s of the converged light relative to the wavelength λ of thelight source and the numerical aperture NA of the converging lens isdetermined by formula (1) given below:

i.e., s=cλ/NA  (1)

When it comes to a laser light having a cross sectional light intensityconforming with, for example, the Gaussian distribution and a diameterin which the light intensity in the edge portion is 1/e² times as highas the light intensity in the central portion, the coefficient c informula (1) is 0.67. In general, the numerical aperture NA of the lensis at most 1. It follows that it is impossible for thediffraction-limited spot diameter s to be smaller than cλ, as apparentfrom formula (1).

As apparent from formula (1), an effective method for obtaining a spotlight having a minimal diameter is to shorten the wavelength λ of thelight source. However, in the case of using a semiconductor laser, theshortening of the wavelength λ is limited. Also, if the wavelength ofthe light source is shorter than that of an ultraviolet light, it isimpossible to use the conventional optical system because of therestriction in the transparent region of the lens material.

On the other hand, as a method for exceeding the limit represented byformula (1), it is proposed to utilize a solid immersion lens (SIL) oran optical near field. The optical near field is generated when a laserlight passes through a circular aperture mounted at the light-emittingedge and having a diameter smaller than the wavelength λ of the lightsource. The optical near field thus generated is utilized by disposing adisc plane in the vicinity of the aperture. To be more specific, a laserlight is formed into an optical near field having a diameter smallerthan the diffraction-limited spot diameter s when the laser beam passesthrough the aperture, and the optical near field thus formed is utilizedfor recording information in an optical disc and for reading therecorded information from the optical disc.

However, a serious problem is inherent in the optical near field thatthe throughput efficiency of the optical near field is very low.Specifically, the aperture is formed in general in a plane of a lightabsorption material having a large optical absorption. It should benoted that a material having a very large absorption loss and athickness large enough to inhibit the light transmission such as a metalmaterial is used as the light-absorbing material so as to inhibit thelight transmission in regions other than the aperture.

When passing through the aperture of the light-absorbing material, thelaser light is absorbed by the light-absorbing material in the vicinityof the aperture, with the result that the laser light intensity isrendered insufficient on the emission side. In other words, thethroughput efficiency of the optical near field is very low and, thus,it is impossible to use the optical near field for the opticalrecording/reading.

On the other hand, it is conceivable to use a high power laser as ameasure for making up for the low throughput efficiency of the opticalnear field. However, in the construction that a light-absorbing materialis mounted on the facet of a high power laser, the temperature in thevicinity of the facet is markedly elevated by the heat generation causedby the light absorption so as to deteriorate the laser facet. It followsthat this measure is not practical.

As described above, in the conventional semiconductor laser device inwhich a small aperture is formed in the light-emitting surface, theefficiency for the laser light to pass through the aperture is very low,making it impossible to use the conventional semiconductor laser devicefor the optical recording. On the other hand, if a high power laser isused as a measure against the low throughput efficiency, the temperaturein the vicinity of the laser facet is markedly elevated so as todeteriorate the high power laser.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor laserdevice capable of minimizing the spot diameter of the laser light andhigh in the optical throughput efficiency through an aperture.

The present invention has been achieved on the basis of the propertiesfound by the present inventors that, when the aperture width is verysmall, the degree of light absorption differs depending on thepolarizing direction of the laser light. It should be noted that thetechnical idea of the present invention resides in that the direction ofthe short aperture width of a small aperture is set parallel to thepolarizing direction of the semiconductor laser element so as to improvethe optical throughput efficiency through the aperture.

To be more specific, the present invention provides a semiconductorlaser device comprising a semiconductor laser element and alight-absorbing film having an aperture formed on the outside of thelight-emitting surface of the semiconductor laser element, characterizedin that the aperture is formed such that the aperture width W₁ in adirection parallel to the polarizing direction of the laser light issmaller than the aperture width W₂ in a direction perpendicular to thepolarizing direction.

The present invention also provides a semi-conductor laser devicecomprising a semiconductor laser element and a light-absorbing filmhaving an aperture formed on the outside of the light-emitting surfaceof the semiconductor laser element, wherein the aperture is formed suchthat the aperture width W₁ in a direction parallel to the polarizingdirection of the laser light is smaller than half the oscillationwavelength of the semiconductor laser element, and the aperture width W₂in a direction perpendicular to the polarizing direction is larger thanthe aperture width W₁.

The semiconductor laser devices according to preferred embodiments ofthe present invention are featured mainly as follows:

(a) A dielectric film is arranged between the light-emitting surface andthe light-absorbing film. It is possible for the dielectric film to beformed, as desired, to fill the aperture.

(b) The aperture width of the aperture in a direction parallel to thepolarizing direction of the laser light is set to fall within a range inwhich the absorption loss of the laser light is made smaller by at leastone place than that in the case where an aperture of the same width isformed to extend in a direction perpendicular to the polarizingdirection of the laser light.

(c) The width of the aperture in a direction parallel to the polarizingdirection of the laser light is shorter than one-third of theoscillating wavelength of the semiconductor laser element.

(d) The semiconductor laser element is of an edge-emitting type and hasan oscillation mode of TM mode.

(e) The light absorption film is made of a metal.

(f) An insulating film is arranged between the light-emitting surfaceand the light-absorbing film, and the optical thickness of theinsulating film falls within a range of between 0.05λ and 0.35λ relativeto the oscillating wavelength λ.

The present inventors have found that the loss of the laser light in thesmall aperture is dependent on the polarizing direction of the laserlight and on the shape of the aperture. To be more specific, the loss isincreased if the aperture width in a direction perpendicular to thepolarizing direction of the laser light is narrowed, and the loss is notincreased even if the aperture width is narrowed in a direction parallelto the polarizing direction of the laser light. The specific reason forthis principle will be described herein later.

It follows that, if the aperture width W₁ in a direction parallel to thepolarizing direction of the laser light, the aperture being formed onthe outside of the light-emitting surface of the semiconductor laserelement, is made smaller than the aperture width W₂ in a directionperpendicular to the polarizing direction of the laser light as in thepresent invention, it is possible to obtain a spot light having asmaller diameter without increasing the absorption loss around theaperture. It follows that it is possible to minimize the spot diameterof the laser light so as to obtain a semiconductor laser device having ahigh light transmission efficiency through the aperture.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1A is an oblique view schematically showing the construction of asemiconductor laser device according to a first embodiment of thepresent invention;

FIG. 1B is a cross sectional view schematically showing the constructionof the semiconductor laser device in the first embodiment of the presentinvention;

FIGS. 2A and 2B show examples of calculating the waveguide mode within ametal waveguide in the first embodiment of the present invention;

FIG. 2C schematically shows the light intensity distribution forexplaining the calculation examples shown in FIGS. 2A and 2B;

FIG. 2D schematically shows the relationship between the aperture andthe waveguide mode;

FIG. 3A shows the shape of the aperture and the light intensitydistribution in each direction;

FIG. 3B shows in the form of a three dimensional distribution the lightintensity distribution shown in FIG. 3A;

FIG. 4 shows an example of analysis of the waveguide mode for Ag in thefirst embodiment of the present invention;

FIG. 5 shows an example of calculating the dependence of the waveguidemode loss in a Au waveguide on the aperture width W in the firstembodiment of the present invention (λ=650 nm);

FIG. 6 shows an example of calculating the dependence of the waveguidemode loss in a Ag waveguide on the aperture width W in the firstembodiment of the present invention (λ=650 nm);

FIG. 7 shows an example of calculating the dependence of the waveguidemode loss in a Cu waveguide on the aperture width W in the firstembodiment of the present invention (λ=650 nm);

FIG. 8 shows an example of calculating the dependence of the waveguidemode loss in an Al waveguide on the aperture width W in the firstembodiment of the present invention (λ=650 nm);

FIG. 9 shows an example of calculating the dependence of the waveguidemode loss in a Pt waveguide on the aperture width W in the firstembodiment of the present invention (λ=650 nm);

FIG. 10 shows an example of calculating the dependence of the waveguidemode loss in a Ti waveguide on the aperture width W in the firstembodiment of the present invention (λ=650 nm);

FIG. 11 shows the light intensity distribution after the light isemitted from the aperture into the air atmosphere in the firstembodiment of the present invention;

FIG. 12 shows the state of the waveguide mode relative to a Au aperturein the first embodiment of the present invention, covering the casewhere the light source wavelength is 400 nm;

FIG. 13 shows the state of the waveguide mode relative to a Al aperturein the first embodiment of the present invention, covering the casewhere the light source wavelength is 400 nm;

FIG. 14 shows an example of calculating the dependence of the waveguidemode loss in a Au waveguide on the aperture width W in the firstembodiment of the present invention (λ=400 nm);

FIG. 15 shows an example of calculating the dependence of the waveguidemode loss in an Al waveguide on the aperture width W in the firstembodiment of the present invention (λ=400 nm);

FIG. 16 is an oblique view showing schematically showing theconstruction of semiconductor laser device according to a secondembodiment of the present invention;

FIG. 17 is an oblique view showing schematically showing theconstruction of semiconductor laser device according to a thirdembodiment of the present invention;

FIG. 18 shows an example of calculating the dependence of a reflectanceR on the thickness d of an insulating film in the third embodiment ofthe present invention; and

FIG. 19 is an oblique view schematically showing the construction of asemiconductor laser according to a modification of the third embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in detail with reference tothe embodiments shown in the accompanying drawings.

First Embodiment

FIG. 1A is an oblique view schematically showing the construction of asemiconductor laser device according to a first embodiment of thepresent invention. As shown in the drawing, the semiconductor laserdevice comprises a semiconductor laser element 10 having an active layer11 formed therein and also having a facet to which a light-emittingplane belongs protected by an insulating film 12, and a light-absorbingfilm 13 having an aperture 14 and mounted to the semiconductor laserelement 10.

The aperture 14 is formed in a position facing a part of thelight-emitting plane and serves to transmit a part of the laser lightemitted from the semi-conductor laser element. To be more specific, theaperture 14 serves to form the intensity distribution of the laser lightinto a width smaller than the oscillating wavelength λ when the laserlight emitted from the semiconductor laser element 10 passes through theaperture 14. Also, in the drawing, the aperture 14 is shaped oblong inwhich each of the four corners of the aperture 14 in a planar shape isarcuate. Alternatively, it is possible for the aperture 14 to be shapedrectangular in which each of the four corner portions in a planar shapehas a right angle. The aperture 14 may be a slit. The construction ofthe aperture 14 described above is also employed in any of theembodiments described below.

The semiconductor laser element 10 is oscillated in a TE mode, and thepolarizing direction of the laser light is in parallel to the junctionplane, i.e., the direction of the electric field vector is equal to ahorizontal direction.

FIG. 1B is a cross sectional view showing as an example the constructionof the semiconductor laser element 10. As shown in the drawing, thesemiconductor laser element 10 comprises an n-type GaAs substrate 101,an n-type InGaAlP clad layer 102, an active layer 103 (11) comprising anInGaAlP multilayered quantum well structure (MQW), a p-type InGaAlP cladlayer 104, an n-type GaAs current blocking layer 105, and a p-type GaAscontact layer 106, which are laminated one upon the other in the ordermentioned. Also, the clad layer 104 is formed into a mesa shape and thecurrent blocking layers 105 are embedded on the both side regions so asto form a ridge stripe laser. Where the active layer 11 comprising theMQW (103), the active layer 11 is of a laminate structure comprisingwell layers each having a thickness of several nanometers and barrierwall layers each having a thickness of several nanometers, which arealternately laminated one upon the other. Also, where the active layer11 is in the form of a bulk, the active layer 11 has a thickness of, forexample, 0.05 to 0.06 μm. Needless to say, the cross sectionalconstruction of the semiconductor laser element 10 is not limited thatshown in FIG. 1B, and it is possible to employ various modifications.

It is possible to use, for example, SiO₂, TiO₂, Ta₂O₅, and α-Si, forforming the insulating film 12. The insulating layer 13 belongs to thesemiconductor laser element 10 and plays the role of the protective filmof the facet of the semiconductor laser element and the role ofpreventing the short circuit between the n-type semiconductor layer andthe p-type semi-conductor layer caused by the light-absorbing film 13.

It is possible to use a metal such as Au, Ag, Cu, Al, Pt or Ti forforming the light-absorbing film 13. Since a metal has a very largeabsorption coefficient, the light is substantially prevented from beingtransmitted through the light-absorbing film 13 except the apertureregion, if the light-absorbing film 13 has a thickness of, for example,about 100 nm. However, the loss of light passing through the aperturehas not yet been analyzed sufficiently. In an ordinary small aperture,the light passing through the aperture also incurs a large loss, givingrise to the problem that the efficiency is very low.

The present invention is featured in that the aperture width W₁ in adirection parallel to the polarizing direction of the laser light ismade smaller than the aperture width W₂ in a direction perpendicular tothe polarizing direction of the laser light. To be more specific, theaperture width W₁ in a direction parallel to the polarizing direction ofthe laser light is made smaller than half the oscillating wavelength ofthe semiconductor laser, and the aperture width W₂ in a directionperpendicular to the polarizing direction of the laser light is madelonger than the aperture width W₁ noted above. The particularconstruction makes it possible to markedly diminish the loss of thelight passing through the aperture. The principle of the particularfunction will now be described.

Specifically, FIGS. 2A and 2B show an example of calculating thewaveguide mode in a metal waveguide. The calculating example is directedto the light intensity distribution, covering the case where the laserlight having a cross sectional intensity conforming with thedistribution sufficiently larger than the aperture width and emittedfrom the active layer 11 passes through the aperture 14 extendingthrough the insulating film 12 and the light-absorbing film 13 isoutputted to the outside (FIG. 2C).

For brevity, the waveguide nodes were calculated for two cases where thesmall aperture 14 having a width of 50 nm (FIG. 2A) was formed in thelight-absorbing film 13 made of gold (Au) and where the small aperture14 having a width of 300 nm (FIG. 2B) was formed in the film 13.Incidentally, n₁ in the drawings denotes the real part of the complexrefractive index n, and κ represents the extinction coefficient in theimaginary part of the complex refractive index n (n=n₁−iκ=0.15−3.5i).

As shown in the drawings, the TE mode, in which the direction of theelectric field vector is parallel to the boundary Bd between the goldlayer in the longitudinal direction of the aperture 14 and the air (FIG.2D), and the TM mode, in which the direction of the magnetic fieldvector is parallel to the boundary noted above, widely differ from eachother in the distribution shape of the mode. The boundary Bd is locatedat the position 0 on one of outer edges of the aperture 14 as shown inFIG. 2C. Incidentally, the TE mode and the TM mode noted above, whichdiffer from the oscillation mode of the semiconductor laser element 10,denote the laser light in the cross section defined in the boundarydirection (longitudinal direction) of the aperture 14. AS describedpreviously, the oscillation mode is the TE mode.

The light intensity in each of FIGS. 2A and 2B denotes the component inthe propagating direction of the Poynting vector. The discontinuity ofthe light intensity at the interface between the air layer and the metallayer in the TM mode is derived from the discontinuity of the componentof the electric field vector in a direction perpendicular to theboundary face. To be more specific, since the amount of continuity isequal to the product between the component of the electric field vectorin a direction perpendicular to the boundary face and n², the lightintensity is markedly diminished within the metal having a largeabsolute value of the real part of n² by the coefficient of 1/n².

On the other hand, the TE mode is a mode in which the permeation intothe metal layer is large because the electric field of the TE mode isdirected parallel to the boundary and is continuous at the boundarybetween the air and the metal.

If the aperture width is relatively large, the permeation of the TE modeis not prominent as shown in FIG. 2B. However, if the aperture width issmall, the TE mode and the TM mode widely differ from each other in thepermeation rate such that the permeation of the TE mode is renderedprominent, as shown in FIG. 2A.

In the TE mode, the permeating portion into the metal layer incurs alarge absorption loss and, thus, the loss of the waveguide mode is largewhere the aperture width is small. In the TM mode, however, the loss ofthe waveguide mode is very small even if the aperture width is smallbecause the permeation into the metal layer is small as described above.

The embodiment shown in FIGS. 1A and 1B utilizes the feature describedabove. To be more specific, by utilizing the feature that the loss isnot increased even if the aperture width is diminished in a directionparallel to the polarizing direction of the laser light, the aperturewidth W₁ in the direction noted above is decreased so as to make itpossible to obtain a fine spot light with a high efficiency.

FIG. 3A shows the shape of the aperture and the light intensitydistribution in each direction. On the other hand, FIG. 3B shows in theform of a three dimensional distribution the light intensitydistribution shown in FIG. 3A. By setting the shape of the aperture asshown in FIG. 3A in the polarizing direction of the laser light, it ispossible to achieve a waveguide mode having a very small loss even in afine aperture smaller than the wavelength.

The description given above with reference to FIGS. 2A to 2D, 3A and 3Bcovers the case where a fine aperture is formed in an absorption thinfilm made of gold. However, the same effect can also be obtained in thecase where the fine aperture is formed in a thin film made of anothermetal, e.g., silver (Ag). FIG. 4 shows an example of the analysis of thewaveguide mode in the case where the fine aperture is formed in a thinAg film. As apparent from the drawing, the permeation of the TM mode isalso small, leading to a small loss.

FIG. 5 is a graph exemplifying a calculation of the dependence of thewaveguide mode loss in the Au waveguide on the aperture width W. In thegraph of FIG. 5, the absorption coefficient α of the light absorptionfilm 13 for every waveguide mode is plotted as a loss α in the ordinate,with the position in the direction of the aperture width based on thereference position set at the left edge of the aperture 14 being plottedon the abscissa. It is clearly seen from FIG. 5 that, in the region of asmall aperture width, the loss of the TM mode is very much lower thanthat of the TE mode. In other words, the loss of the TE mode is about100 times as high as that of the TM mode. The experimental data given inFIG. 5 quantitatively support that the loss is markedly diminished byusing the TM mode in which the direction of the polarization isperpendicular to the side of the aperture, in respect of the directionof the small aperture width.

To be more specific, where an aperture of 50 nm is formed in a Au filmhaving a thickness d of 100 nm, the transmittance through the lightabsorption film 13 calculated on the basis of λ and α shown in FIG. 5and κ shown in FIG. 2 are as follows:

Transmittance e^((4π/λ)κd) for 0.001 the region outside aperture 14:Transmittance e^(αd) of TE 0.006 mode for aperture 14: Transmittancee^(αd) of TM 0.956 mode for aperture 14:

It follows that the light is sufficiently attenuated when the lightpasses through the light absorption film 13 having a thickness d and, inthe TM mode, the light is capable of passing through the aperture whilesubstantially incurring no loss, which are also shown in FIG. 2C. On theother hand, in the TE mode, the light is scarcely transmitted throughthe aperture having an aperture width of about 50 nm. As apparent fromFIG. 5, the loss can be lowered in the TE mode by setting the aperturewidth at 260 nm or more. It follows that it is possible to obtain a spotlight very low in loss and very small in size, if the aperture is shapedsuch that the width in a direction parallel to the polarizationdirection of the laser light is small and the width in a directionperpendicular to the polarization direction of the laser light is large,as shown in FIGS. 1A or 3A.

In the embodiment shown in FIG. 1A, the aperture 14 is vertically longbecause the laser light is polarized in the horizontal direction. To bemore specific, in the embodiment shown in FIG. 1A, the aperture width W₁in the horizontal direction is smaller than half the wavelength and theaperture width W₂ in the vertical direction is larger than W₁. Theresults of calculation given above clearly support that the loss oflight passing through the aperture is very low even if the aperturewidth W₁ is set at a very small value of 50 nm. It follows that it ispossible to achieve the optical recording/reading with a spot lighthaving a diameter which is one place smaller than the wavelength λ.

Incidentally, the aperture width W₁ is not limited to 50 nm or a valuesmaller than λ/2. For example, on the basis of comparison with the casewhere an aperture of the same width is formed to extend in the verticaldirection (TE mode), it is desirable to set the aperture width W₁ tofall within a range in which the absorption loss relative to the laserlight is diminished by one place, i.e., W₁≦260 nm. To be more specific,it is desirable to set the aperture width W₁ in FIG. 5 at an optionalvalue falling within a range (W≦260 mm) in which the absorption loss αof the TM mode is made smaller than the absorption loss α of the TE modeby at least one order of magnitude on the basis of the relationshipamong the absorption loss α in the TE mode of the light absorption film13 when the direction perpendicular to the polarization direction ismade equal to the direction of the aperture width W, the absorption lossα in the TM mode of the light absorption film 13 when the polarizationdirection is equal to the direction of the aperture width W, and thesize of the aperture width W.

It is also effective to use a metal other than Au used in the embodimentdescribed above for forming the light absorption film 13. FIGS. 6 to 10show examples of calculation of the loss for each of the TE mode and theTM mode in the cases of using Ag, Cu, Al, Pt and Ti, respectively, forforming the light absorption film 13. In any of these cases, the loss inthe TM mode is lower than that of the TE mode. Also, the difference inthe loss is about two orders of magnitude or more in each of using Ag,Cu and Al for forming the light absorption film 13 as in the case ofusing Au for forming the film 13, supporting that the metals exemplifiedabove are highly effective when used for forming the light absorptionfilm.

The light intensity distribution after emission from the aperture 14will now be described. FIG. 11 shows the light intensity distributionafter the light is emitted from the aperture into the air atmosphere.The drawings on the left side of FIG. 11 show the entire light intensitydistribution including the evanescent light, and the drawings on theright side show the propagating light components, i.e., the Poyntingvector components. As apparent from FIG. 11, the propagating lightcomponent is increased with increase in the ratio W/λ of the aperturewidth to the laser wavelength. Since the light actually utilizedconsists mainly of the propagating light component, it is desirable forthe ratio W/λ to be large. On the other hand, it is necessary todiminish the value of W in order to obtain a fine spot. It follows that,in obtaining a fine spot, it is advantageous for the wavelength λ of thelaser light to be short.

On the other hand, where the wavelength of the light source is short, itis necessary to arrange an absorption film adapted for the shortwavelength. FIG. 12 shows the situation of the waveguide mode in thecase where wavelength of the light source is 400 nm in respect of theaperture formed in a gold thin film shown in FIG. 2A. What should benoted is that the permeation for the TM mode is large unlike thesituation shown in FIG. 2A in spite of the fact that the aperture isformed in a gold thin film. The reason for the particular situation isthat, in the wavelength of 400 nm, the absolute value in the real partof n² in a gold film (n₁ ²−κ²=1.552²−1.75²=−0.746) is smaller than 1and, thus, the electric field intensity within the metal film isincreased by the coefficient of 1/n².

On the other hand, the absolute value in the real part of n² in analuminum (Al) film (n₁ ²−κ²=−19.64) is larger than 1 by at least oneorder of magnitude in 400 nm, too. As a result, the TM mode is greatlyattenuated within the metal, leading to a small absorption loss, asshown in FIG. 13.

FIGS. 14 and 15 quantitatively show the situations. As apparent fromFIGS. 14 and 15, it is desirable to use, for example, aluminum forforming the light absorption film for a short wavelength of 400 nm,though it is possible to use an optional material for forming the lightabsorption film 13, as far as the material has an absolute value, whichis larger than 1, in the real part of the square of the refractive index(n₁ ²−κ²) under the oscillation wavelength λ.

According to the embodiment described above, the direction of the shortaperture width W₁ of a fine aperture is set in parallel to thepolarizing direction of the semiconductor laser element 10 so as to makeit possible to provide a semiconductor laser device that makes itpossible to minimize the spot diameter of the laser beam and that has ahigh transmission efficiency of the laser light through the aperture.

Second Embodiment

FIG. 16 is an oblique view schematically showing the construction of asemiconductor laser device according to a second embodiment of thepresent invention. As shown in the drawing, the semiconductor laserdevice comprises a semiconductor laser device 20 having an active layer21 arranged therein and having the facet protected by an insulating film22 and a light absorption film 23 having a aperture 24. The polarizingdirection of the laser is perpendicular to the junction plane, i.e., thedirection of the electric field vector is perpendicular to the junctionplane.

The second embodiment differs from the first embodiment in thepolarizing direction of the laser light. It is possible to achieve alaser in which the polarizing direction is perpendicular to the junctionplane as in the second embodiment by, for example, arranging an opticalabsorbing layer within a clad layer so as to make the loss of the TEmode larger than that of the TM mode, i.e., by arranging a lightabsorption layer in such a position, or by introducing a tensile straininto the active layer 21 so as to achieve a TM mode oscillation.

Since the polarizing direction is parallel to the vertical direction inthe second embodiment, the aperture 24 formed in the light absorptionfilm 23 extends in a lateral direction. In other words, the aperturewidth W₁ in a direction parallel to the polarizing direction is smallerthan half the oscillating wavelength of the semiconductor laser, and theaperture width W₂ in a direction perpendicular to the polarizingdirection is larger than the aperture width W₁. The particularconstruction of the second embodiment is advantageous in that the twodimensional shape in the oscillation mode of the semiconductor laserelement can be made similar to the shape of the aperture. As a result,in addition to the effects produced by the first embodiment, anadditional effect can be obtained that it is possible to allow the lightoutput from the semiconductor laser element 20 to be incident highlyefficiently on the aperture.

Third Embodiment

FIG. 17 is an oblique views schematically showing the construction of asemiconductor laser device according to a third embodiment of thepresent invention. The semiconductor laser device shown in FIG. 17comprises a surface-emitting type semiconductor laser element having anactive layer 32 arranged therein and having the edge surface protectedby an insulating film 36 and a light absorption film 39 having anaperture 40.

The surface-emitting type semiconductor laser element according to thethird embodiment of the present invention comprises an n-type DBRreflection layer 31, an active layer region 32 having an opticalthickness that is equal to the oscillation wavelength, a p-type DBRreflection layer 33 and a p-type contact layer 34, which are laminatedone upon the other on one surface of an n-type semiconductor substrate30.

A p-type contact layer 35 and an insulating film 36 are formed in theorder mentioned on the surface of a part of the p-type contact layer 34,and a p-type electrode 37 is formed on the surface of the other part ofthe p-type contact layer 34. Further, an n-type electrode 38 is formedon the other surface of the n-type semiconductor substrate 30. Thesemiconductor laser element of the particular construction is called avertical-cavity surface-emitting semiconductor laser element.

Still further, a light absorption film 39 having an aperture 40 isformed on the insulating film 36 of the semiconductor laser element.

The aperture width is small (λ/2 or less) in a direction parallel to thepolarizing direction of the laser light and large in a directionperpendicular to the polarizing direction of the laser light in thisembodiment, too, making it possible to decrease markedly the loss oflight as already described in conjunction with the first and secondembodiments.

The thicknesses of the insulating films 12, 22 and 36 used in the first,second and third embodiments, respectively, will now be described. Asalready described, the light can be attenuated sufficiently in theregion other than the aperture if the thickness of each of the lightabsorption films 13, 23 and 39 is about 100 nm. The thickness notedabove corresponds to an optical thickness of about λ (one wavelength).An ordinary metal film having a thickness of the level noted aboveexhibits a reflectance of at least 90% and, thus, is sufficientlyeffective when used as a reflector of a cavity.

However, where a metal film is formed to cover the facet of anedge-emitting type laser as in the first embodiment or the secondembodiment, it is absolutely necessary to form the insulating film 12 or22 between the facet and the metal film. Also, it is necessary to setthe thickness of the insulating film 12 or 22 at a value at which a highreflectance can be ensured because the reflectance is decreaseddepending on the thickness of the insulating film. Incidentally, in theknown semiconductor laser element failing to fall within the scope ofthe present invention, the reflectance of the insulating film need notbe considered. In general, the thickness of the insulating film is setat λ/4.

FIG. 18 is a graph showing an example of calculating the dependence ofthe reflectance R on the thickness d of the insulating film 12 or 22. Inthis calculation, the optical thickness of the gold (Au) film is set atλ (one wavelength). However, since the gold film has a sufficientlylarge absorption coefficient, the dependence of the reflectance R on thethickness of the metal film is small. In the graph of FIG. 18, thereflectance R is plotted on the ordinate, with the optical thickness ofthe insulating film 12 or 22 normalized by the wavelength (nd/λ) plottedon the abscissa. As apparent from FIG. 18, the reflectance R isperiodically increased and decreased. It can be understood from FIG. 18that a high reflectance R is exhibited in the vicinity of the valuedenoted by formula (2):

nd/λ=0.2+0.5 m  (2)

where m represents an integer.

In order to obtain a reflectance R of, for example, at least 95%, dshould be set to satisfy the relationship given by formula (3) below:

0.05+0.5 m≦nd/λ≦0.35+0.5 m  (3)

Incidentally, since the insulating film sufficiently plays the role of aprotective film even if the film is thin, it suffices to set the valuend/λ of the insulating film 12 or 22 to fall within the range defined informula (4) given below:

0.05≦nd/λ≦0.35  (4)

As described above, according to the third embodiment, the effectsimilar to that produced by the first embodiment can be obtained inrespect of the vertical-cavity surface-emitting type semiconductor laserdevice, too.

The present invention is not limited to each of the embodimentsdescribed above.

To be more specific, the semiconductor laser element in each of theembodiments described above is not limited to the element formed of thematerial specified in each of these embodiments. For example, it ispossible to use various materials in the present invention includingInGaAsP series, GaAlAs series, InGaAlP series, InGaAlN series, InGaAlBNseries, InGaAsSb series, CdZnMgSSe series materials, etc.

Also, the laser structure is not limited to the structures shown inFIGS. 1A, 1B, 16 and 17, and the laser structure can be modifiedappropriately. For example, it is possible to use an insulating sapphiresubstrate 50 in place of the semiconductor substrate, as shown in FIG.19. In the laser structure shown in FIG. 19, an n-type GaN layer 51, ann-type GaAlN clad layer 52, an MQW active layer 53 made of InGaN, ap-type GaAlN clad layer 54, a p-type GaN contact layer 55 and a ridgeportion 56 are laminated one upon the other in the order mentioned onthe sapphire substrate 50. Also, the laser facet (light-emittingsurface) is covered with an insulating film 57.

In the case of the example shown in FIG. 19, the sapphire substrate 50is an insulator. Therefore, each of the layers 52 to 55 is partly etchedin the etching step until the etching proceeds to reach the clad layer51, and an n-side electrode (not shown) is formed on the exposed cladlayer 51. Also, a p-type electrode (not shown) is formed to cover thep-type GaN contact layer and the ridge portion 56. It should also benoted that a light absorption film 58 having an aperture 59 is formed onthe insulating film 57 of the semiconductor laser element of themodification shown in FIG. 19. The aperture 59 is positioned to face apart of the light-emitting surface as in the embodiments describedpreviously.

Incidentally, the inner space of each of the apertures 14, 24, 40 and 59formed in the first to third embodiments is filled with air.Alternatively, it is possible for the inner space of the aperture to befilled partly or entirely with the insulating film 12, 22, 36 or 57.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A semiconductor laser device, comprising: asemiconductor laser element having a light-emitting surface from which alaser light having a wavelength λ is emitted; and a light absorptionfilm covering the light-emitting surface of said semiconductor laserelement, the film having a slit for passing through the light emittedfrom the light-emitting surface, the slit having a first width W1 in adirection parallel to the polarizing direction of said laser light and asecond width W2 in a direction perpendicular to the polarizing directionof the laser light; wherein the first width W1 is shorter than half thewavelength λ of semiconductor laser element and also is shorter than thesecond width W2.
 2. The semiconductor laser device of claim 1, whereinthe first width W1 is in a direction parallel to a longitudinaldirection of an emitted-light pattern of said laser light.
 3. Thesemiconductor laser device of claim 1, wherein the light absorption filmis made of a thin metal.
 4. The semiconductor laser device of claim 3,further comprising an insulating film between the light-emitting surfaceof the semiconductor element and the light absorption film.
 5. Thesemiconductor laser device of claim 4, wherein the slit is filled partlyor entirely with the insulating film.
 6. The semiconductor laser deviceof claim 1, wherein the slit is filled with air.
 7. A semiconductorlaser device, comprising: a semiconductor laser having a light-emittingsurface from which a polarized laser light is emitted; a protective filmformed on the light-emitting surface of the semiconductor laser; and ametal film formed on the protective film and covering the light-emittingsurface of said semiconductor laser, the metal film having an aperturefor passing through the polarized laser light, the aperture having afirst width W1 in a direction parallel to the polarizing direction ofthe laser light and a second width W2 in a direction perpendicular tothe polarizing direction of the laser light; wherein the first width W1is shorter than half the wavelength λ of the laser light and also isshorter than the second width W2.
 8. The semiconductor laser device ofclaim 7, wherein the first width W1 of the aperture is 50 nm.
 9. Thesemiconductor laser device of claim 7, wherein the thickness of themetal film is about 100 nm.
 10. The semiconductor laser device of claim7, wherein the thickness of the metal film is about one wavelength λ ofthe laser light.
 11. The semiconductor laser device of claim 7, whereinthe thickness of the protective film is about one-fourth of wavelength λof the laser light.
 12. The semiconductor laser device of claim 7,wherein the metal film is made of one selected from the group consistingof Ag, Cu, Pt and Ti.
 13. The semiconductor laser device of claim 7,wherein the metal film is made of Au or Al.